Guiding a farm tractor, construction vehicle, mining truck, army tank or other large vehicle according to a precise, desired path often saves time and energy compared to sloppy operating. Precise farm tractor control, for example, benefits farmers, consumers and society as a whole. Farmers are able to work more efficiently, and spend less money on fertilizers and pesticides. Consumers enjoy lower prices for high quality produce, and precise use of farm land and farm chemicals reduces waste and excess runoff.
Autopilot systems that guide farm tractors, spray trucks, harvesters and the like with high accuracy and repeatability contribute to efficient land and chemical use. For example, fields are often sprayed with booms 90-feet-wide or even larger. When using such a wide boom it would seem prudent to allow a few feet of overlap from one spray swath to the next. However, the overlap may be reduced to just inches if the tractor or spray truck is equipped with a high performance autopilot. The savings accrued from not double-spraying swath edges add up quickly on large area farms.
Existing autopilot systems are adequate for keeping vehicles on a predefined path. These autopilots are based on feedback control techniques and they can make a large farm tractor, for example, follow a line within one or two inches cross-track error. FIG. 1A shows a vehicle following a path under feedback control; vehicle 105 follows path 110. When the cross track error, XTE, is smaller than the vehicle wheelbase, b, the vehicle is in a small signal regime or “close” to the desired path.
Feedback autopilots do not perform as well when a vehicle is far off from a desired path. Common situations where this occurs include joining a path from far away or turning around at the end of a path to join a nearby path. FIG. 1B shows a vehicle with a possible path 125 that could result from using a small-signal feedback autopilot in a large-signal regime. Vehicle 115 is guided by a feedback autopilot toward path 120. When the cross track error is larger than the vehicle wheelbase, the vehicle is in the large signal regime or “far” from the desired path.
With conventional feedback autopilots, tradeoffs exist between performance in the small signal regime and acceptable behavior in the large signal regime. For example, high feedback gain that keeps cross track error small in the small signal regime may result in steep approaches to a desired path and oscillation when joining the path as shown in FIG. 1B. Conventional autopilots avoid undesirable large-signal behavior by invoking heuristic limits when large deviations from a desired path are encountered. These limits mean that large-signal guidance is not as efficient as it could be.
In the examples above wheelbase is used as an example of a characteristic length scale to divide small and large signal regimes. Other characteristic lengths may be used; e.g. distance travelled in a characteristic autopilot response time. More generally, the small signal regime refers to any situation where a vehicle autopilot behaves as a linear, time-invariant system. The large signal regime, on the other hand refers to situations where non-linear behavior, such as steering angle limits or steering angle rate limits, occurs.
What is needed is a vehicle autopilot that offers high performance guidance regardless of whether a vehicle is near or far from a desired path. The autopilot should not only keep a vehicle on a path, but also guide it efficiently to join a path from any starting point.